Brushless permanent magnet (PM) machines use a rotor in which permanent magnets are embedded, arranged so that multiple magnetic poles project from the rotor radially, axially or transversely. The stator contains electromagnets which generate magnetic poles that move around the perimeter of the stator. The attraction and repulsion of the moving poles created by the current through the stator windings to the magnetic poles created by the PMs embedded within the rotor create torque.
Induction motors, on the other hand, don't include magnets on the rotor but instead include loops of conductors, e.g., “coils”. In the presence of the moving poles created by flux created by current through the stator windings, the changing magnetic flux generated by the stator windings induce currents in the conducting loops, which gives rise to opposing magnetic fields. The attraction and repulsion between the moving stator fields and the magnetic fields induced on the rotor coils create torque. Because the topology of the loops of coils located around the rotor resembles a squirrel cage in common construction, the conductive structure is commonly referred to as a “cage”.
Some machines use permanent magnets on the rotor but also include a cage structure to provide additional torque at start-up. For example, these machines may be called line start permanent magnet machines. Also, field wound synchronous machines commonly have loops of conductive material that circumscribe and surround the poles of the rotor, to aid startup of the machine, as well as dampen transient behavior on the machine and electrical network. The additional magnetic fields produced by the cage(s) is(are) proportional to the current in the cage loops, which is proportional to the change of flux imposed on the cage loop. When there is large relative difference in speed between rotor and stator fields, the change in magnetic flux seen by a cage loop is relatively high, which induces a relatively large current and produces a relatively large opposing magnetic pole. The stronger magnetic attraction and repulsion that is created between the magnetic poles of the rotor and the magnetic poles of the stator manifests itself as additional torque. The resultant torque tends to force the rotor and stator to synchronize in speed. As the relative speed between rotor and stator decreases, the current induced in the cage loops becomes smaller, as does the additional magnetic flux contributed by the cage. If the speed of the rotor is synchronous to that of the fields created by the stator windings, the current in the cage(s) becomes zero, and the cage stops providing the benefit of additional magnetic flux, as well as torque that would restore synchronism between stator and rotor. At this point, the rotor is operating using only the attraction and repulsion between the PMs on the rotor and the moving poles created by the stator windings.
While a wound field synchronous machine may be directly connected the power network without an intermediate power electronic converter, a PM machine without a cage requires a power electronic converter. This power electronic converter acts as a medium between the fixed amplitude and frequency of the power network and that of the voltage amplitude and frequency of the synchronous machine being driven, so that the mechanical system being driven by the motor or generator may be controlled in an effective manner. (The control of the field current in the wound field synchronous machine allows control of the amplitude of the stator voltage, the permanent magnet machine does not allow this type of control.)
A drawback of the power electronic converter is that there tends to exist variations in the current waveforms between the converter and machine being driven. These variations in the current waveforms will no longer allow the poles of the stator and of the rotor to rotate in a synchronous manner. These variations in current are often called time harmonics, and will create losses and torque pulsations in the machine. The torque pulsations may be reduced by providing the rotor pole with a conducting cage. While the conductive cage acts to dampen torque pulsations, the resultant currents which create the damping torques in the cages create losses, which are not preferred under normal operation.
Due to these losses, cages are not preferred in PM machines, as the magnets are quite sensitive to the temperature rise from the losses. The magnets are selected based on temperature and field requirements. Magnets that are capable of operating at higher temperatures and lower fields are more expensive, and have lower remanence, due to the inclusion of Dysprosium, in particular. In this way, it is preferred to have a rotor that is cooler or protects the magnets from low field levels. The low field levels are not usually a difficulty when under normal operation, but become limiting when a transient event occurs, such as a short circuit.
There are a number of advantages to having a combination of permanent magnet and cage. For example, a cage can moderate the effects of sudden changes of magnetic flux, i.e., flux linkage, and thus can smooth changes in rotor speed, dampen the effects of ripple currents in the stator windings, etc. For line-start synchronous machines, the cage provides additional torque during startup. There are disadvantages to having a cage, as well. The presence of a cage induces a loss that reduces the efficiency of a PM machine and results in non-trivial rotor heating. Because of this, brushless PM machines typically do not include a cage of any kind.
FIG. 1A is a section of a rotor of a conventional permanent magnet electrical machine without a protective cage. Rotor 100 contains permanent magnets 102 embedded within the rotor body to create a pattern of alternating north (N) and south (S) magnetic poles around the outer diameter of the rotor. In the conventional rotor 100 shown in FIG. 1A, the magnets are positioned so that the magnetic field produced by each magnet has a vector that is normal to the outer surface of rotor 100.
FIG. 1B is a section of another rotor of a conventional permanent magnet electrical machine without a protective cage. In the conventional rotor 100 shown in FIG. 1B, permanent magnets 102 are arranged in pairs to produce a pattern of alternating north (N) and south (S) magnetic poles around the outer diameter of the rotor. Each pair of magnets is arranged in a V shape such that the magnetic field produced by each pair has a vector that is normal to the outer surface of rotor 100.
Because neither of the conventional rotors shown in FIGS. 1A and 1B include a conducting cage structure, the permanent magnets contained within these conventional rotors are susceptible to damage in response to being subjected to low magnetic flux density in the magnetized direction, such as may occur during startup conditions or during fault conditions. However, a large change in magnetic flux induces voltage in and around the rotor, which when constructed with conductive elements, allow significant currents to flow, which act to create a countervailing magnetic flux and thus protect the permanent magnet from being demagnetized. The eddy currents within the body of the magnet tend to be smaller than the eddy currents at the periphery of the magnet, and due to geometry and magnetic fringing, the countervailing magnetic flux within the body of the magnet tends to be larger than the countervailing magnetic flux at the periphery of the magnet. As a result, the periphery of the magnet has less protection that the rest of the magnet body, and thus the edges of the magnet tend to suffer more demagnetization than is suffered by the core of the magnet. A coupled effect is that the eddy currents create significant additional losses, and within the short circuit time frame, this energy does not propagate throughout the magnet, so the periphery of the magnet exhibits significant temperature rise.
FIG. 2A is a cross section of a rotor 100 having magnets 102 arranged in a V shape showing damage caused by exposure of the magnets to strong magnetic fields. A stator 200 has coils 202 for generating a set of moving magnetic poles. FIG. 2A shows that a portion of magnets 102 have been demagnetized as a result of exposure to large changes of magnetic flux. The shaded portions 204 indicate parts of the magnet that tend to become demagnetized easier than other regions. Magnets that include NdFeB, Ferrite, SmCo, or Alnico, for example, are susceptible to such demagnetization.
FIG. 2B is a graph that illustrates the conditions upon which a magnet becomes demagnetized, shown as an inflection point, or “knee” in the graph. The point at which demagnetization occurs depends upon temperature of the magnet, and the flux density that passes through the magnet. The presence of a cage allows for a minimization that in the reduction of flux density in the magnet that occurs during transient type events, specifically an accidental or purposeful short circuit of one or more of the stator windings. The machine design may be so that under normal operation, the magnet operating point of 110 [C] and 0.4 [T] is obtained. Without a cage, the flux density in portions of the magnet may be reduced to 0.0 [T], for example. As the flux density at 110 [C] is below the knee of about 0.19 [T], these portions of the magnet that fall below the knee are demagnetized. The presence of a cage prevents the flux density from falling below the knee, thus preventing the demagnetization of the magnet.
Some conventional electric machines, such as synchronous machines for example, include cage structures. If the conducting cage is a closed circuit that surrounds the magnetic pole created by the permanent magnet, a change in magnetic flux linked in that cage induces a current in the loop, which creates its own countervailing magnetic field.
Thus, a PM electrical machine with a conductive cage has greater protection from damage to the permanent magnets caused by changes in magnetic flux, but suffers a loss of efficiency during normal operation. A PM electrical machine without a conductive cage has greater efficiency during normal operation, but is susceptible to damage to the permanent magnets during startup or fault conditions.
Accordingly, in light of these disadvantages associated with both machines with cages and machines without cages, there exists a need for PM machines with cages that can be controlled such that the cage conducts when needed and does not conduct when needed, i.e., PM machine with a hybrid cage and methods for operating same.